Aqueous dispersions of copolymers of vinyl acetate and ethylene are known. Typically, copolymers are considered high ethylene copolymers when they have an ethylene content in excess of 40% by weight and often greater than 50% by weight of the polymer. They are commonly referred to as EVA copolymers whereas those having less than 50% ethylene by weight are typically referred to as VAE copolymers. One method for preparing the high ethylene content copolymer emulsions is to polymerize vinyl acetate and ethylene in the presence of a solvent and then mechanically emulsify the resulting polymer. Another method for forming copolymers of vinyl acetate and ethylene is to effect emulsion polymerization of vinyl acetate and ethylene under high pressure in the presence of water and a stabilizing medium.
Many uses have been ascribed to EVA polymers including use as coatings, pressure sensitive adhesives, paper saturation binders, repulpable paper coatings, adhesive binders, and so forth. One of the suggested uses for EVA polymers is in heat seal applications. Aqueous emulsions are coated onto a substrate, e.g., paperboard stock, and dried and often these substrates are rolled for storage and transport. At the site of manufacture of finished products, the paperboard stock having the polymer adhered thereto is affixed or laminated to a surface and heated sufficiently to melt the polymer and effect a seal.
To have good market acceptance for use in heat seal adhesive applications the polymers should have a proper balance of heat seal adhesive characteristics and non-block characteristics. Ideally, a good heat seal adhesive should be capable of forming a good bond at moderately low temperatures, e.g., less than 109° C., preferably less than 99° C. This affords the convertor the ability to run rapid line speeds without the need for excessive heat. Lower temperatures also reduce the risk of operator injury due to burns and minimize substrate damage that would occur at higher temperatures. A good heat seal adhesive must remain non-blocking at ambient temperatures, i.e., those temperatures experienced in non-climate controlled warehouses and trucks, e.g., 35° C. and up.
Two additional requirements for polymers employed in heat seal applications are clarity and hot green strength. Clarity is required in applications such as in heat-sealing of blister pack films. The substrate films are clear and heat seal adhesives that form opaque bonds or lead to discoloration are unsuited for this application. Hot green strength is defined as the strength of the bond as judged by peel adhesion immediately after forming the bond in the heat sealing process. If a polymer does not exhibit hot green strength, the adhesive bond could fail as the part exits the heat sealer and becomes subjected to stresses as it moves along the production line.
Representative processes for producing VAE and EVA polymers are disclosed in the following patents.
U.S. Pat. No. 3,226,352 discloses the batchwise polymerization of ethylene in the presence of water and a nonionic emulsifier system containing an alkylphenoxy polyoxyethylene ethanol emulsifier. The products are characterized by good stability and are odorless, colorless, milky, low viscosity liquids. Emulsifier levels of 15 to 20% by weight relative to total polymer are used to produce solids levels in excess of 25%. The emulsions are reported to be useful in textile finishing, polishes, printing inks, and surface coatings.
U.S. Pat. No. 3,436,363 discloses a batchwise and a continuous process for the polymerization of ethylene in the presence of a polymerizable ethenoid unsaturated mono or polybasic carboxylic acid or sulfonic acid and water. Representative polymerizable ethenoid acids include acrylic, methacrylic, crotonic, itaconic, and the like. The solids level of the polymer emulsions is about 25 to 30% by weight. The emulsions are reported to be useful in paper coating, textile finishing, polishes, and surface coatings.
U.S. Pat. No. 3,355,322 discloses a batchwise process for forming vinyl acetate-ethylene emulsions containing from 20 to 70%, typically 25 to 60% ethylene for use as coatings for paper or as surface coatings. Polymers having an ethylene content of about 30% by weight precipitated on addition of a saturated aqueous salt solution of sodium bicarbonate and were found to have a melt index of 155 g per 10 minutes at a temperature of 190° C.
U.S. Pat. No. 3,692,723 discloses a process for producing heat melting adhesives by the emulsion polymerization of vinyl acetate and ethylene having 30 to 98% ethylene in the presence of poly(vinyl alcohol), a nonionic surfactant having an HLB of from 16–21 and at least one anionic surfactant selected from the group consisting of a sodium, potassium or ammonium salt of lauryl alcohol sulfuric acid ester or a sodium salt of dodecylbenzenesulfonic acid.
U.S. Pat. No. 4,128,518 discloses a process for producing pressure sensitive adhesives comprising an aqueous emulsion of vinyl acetate-ethylene polymer having an ethylene content of 15 to 40% by weight. It is reported that when the ethylene content is above 40% by weight, crystallizability of ethylene appears and the pressure sensitive adhesive is hard and has inferior tackiness and adhesion force.
U.S. Pat. No. 5,747,578 discloses a copolymer in dispersion or redispersible powder form which is stabilized by a protective colloid. It has a Tg of −40 to 10° C. and an ethylene content of at least 30% by weight. The copolymer is prepared by copolymerizing vinyl acetate and ethylene under conditions for forming a first polymer phase having greater than 40% by weight of ethylene and then forming a second copolymer phase, which includes vinyl acetate and having up to 40% by weight of ethylene, based on total amount of monomers. The copolymers are reported to be useful as binders in building material formulations.
U.S. Pat. No. 5,872,181 discloses aqueous vinyl acetate-ethylene polymer adhesives for use in packaging applications and for bonding difficult to bond surfaces. The polymers contain 55–80 wt % vinyl acetate, 15–45 wt % ethylene, and 0 to 30 wt % one or more other ethylenically unsaturated copolymerizable monomer. The polymers are produced by emulsion polymerization using a stabilizer system of poly(vinyl alcohol) as a protective colloid and a surfactant.
U.S. Pat. No. 6,319,978 discloses pressure sensitive vinyl acetate—ethylene-carboxyl based adhesives comprising a high (45–55 wt %) ethylene content emulsion polymer having a Tg of from −25 to −70° C. The high ethylene content emulsion are formulated to be tacky to the touch at room temperature.
WO 02/42342 (2002) discloses ethylene-vinyl acetate emulsion polymers having at least 55% by weight ethylene, which is water dispersible, yet non-dispersible in an aqueous solution containing 0.5% or more of an inorganic salt. The emulsion polymers are stabilized with a hydrophilic polymeric colloid, e.g., polyacrylic acid. The polymers are reported to be useful in producing environmentally friendly products, such as flushable hygienic products. The high ethylene content emulsion is designed to be tacky to the touch at room temperature.
WO 02/079,270 discloses high ethylene/vinyl acetate copolymers formed by polymerizing ethylene and vinyl acetate in the presence of a hydrophilic colloid without a carboxylic acid monomer. The high ethylene content emulsion is designed to be tacky to the touch at room temperature.
JP2002322216A discloses a method for forming an ethylene-vinyl acetate polymer using a redox system where ethylene is the main component. The polymer is formed by emulsion polymerization in the presence of poly(vinyl alcohol). Persulfate and sodium hydrogen sulfite is used as the initiating system. The vinyl acetate content was maintained at a level of about 3% by weight during polymerization.